Add VSCode extension broker

[garagatyi]

What does this PR do?

VS Code extension broker.

Dockerfile for container image:

FROM wsskeleton/theia-endpoint-runtime
WORKDIR /usr/local/bin
RUN wget https://storage.googleapis.com/kubernetes-release/release/v1.8.11/bin/linux/amd64/kubectl && chmod +x ./kubectl
RUN mkdir /.kube && chmod g+w /.kube

Example of meta.yaml for kubernetes extension:

id: vscode-k8s-ext
version: 0.0.1
type: Theia plugin
name: Vscode k8s plugin
title: k8s extension
description: k8s extension
icon: https://www.eclipse.org/che/images/ico/16x16.png
attributes:
  extension: "vscode:extension/ms-kubernetes-tools.vscode-kubernetes-tools"
  container-image: "garagatyi/remotetheia:kubectl"

What issues does this PR fix or reference?

[garagatyi]

@ibuziuk FYI

[ibuziuk]

@garagatyi any instructions how we could try a new broker ?

[garagatyi]

@ibuziuk Full-blown approach would be hard to try. I suppose that not everything is ready to handle this situation, but I'm not sure. I tried a partial approach with a custom main method that parses a local config.json file.
I also tried it with Theia but extension didn't work properly even though the kubernetes panel was added to Theia. I think it was considered as a local plugin rather than a remote one.

To test you'll need to:

1. Apply this PR Add attributes to Che 7 plugin meta; make URL field not mandatory che#12148
2. Build the broker.
3. Add a plugin to plugin registry WIP Add VS Code k8s extension che-plugin-registry#66
4. Create workspace with this plugin enabled and start it.
5. Login to kubernetes using kubectl command from a terminal to create kube context.

[sleshchenko]

Generally looks OK for me.

Please take a look and fix/answer my inline comments and notify me - will do final review.

[garagatyi]

@ibuziuk @sleshchenko Please, review

[sleshchenko]

LGTM

[ibuziuk]

@garagatyi are those instructions up-to-date - #19 (comment) or there is now better approach for trying this PR ?

[garagatyi]

@ibuziuk yes, but kubectl extension won't work in any case. But you can try something else, like Sonarlint

[garagatyi]

@ibuziuk @skabashnyuk @AndrienkoAleksandr please provide review if you will